For a number of known reasons, spinal fixation devices are used in orthopedic surgery to align and/or fix a desired relationship between adjacent vertebral bodies. Such devices typically include a spinal connector, such as a relatively rigid fixation rod, that is coupled to adjacent vertebrae by attaching the connector to various anchoring devices, such as hooks, bolts, wires, or screws. The connector may have a predetermined contour that has been designed according to the properties of the target implantation site, and once installed, the instrument holds the vertebrae in a desired spatial relationship, either until healing or spinal fusion has taken place, or for some longer period of time.
Spinal connectors can be anchored to specific portions of the vertebrae. Since each vertebra varies in shape and size, a variety of anchoring devices have been developed to facilitate engagement of a particular portion of the bone. Pedicle screw assemblies, for example, have a shape and size that is designed and configured to engage pedicle bone. Such screws typically include a threaded shank that is adapted to be threaded into a vertebra, and a head portion having a receiving element, usually in the form of a U-shaped head. A set-screw, plug, or similar type of fastening mechanism is used to lock the spinal connector, e.g., a spinal rod, into the receiving head of the pedicle screw. In use, the shank portion of each screw is threaded into a vertebra, and once properly positioned, a rod is seated through the receiving member of each screw. The rod is locked in place by tightening a cap or other fastener mechanism to securely interconnect each screw and the spinal rod.
Minimally-invasive devices and methods for implanting spinal fixation devices are advantageous because such devices and methods utilize fewer and/or smaller incisions for introducing and implanting anchoring devices and spinal connectors at a target site within a patient's spine. Such procedures offer advantages over invasive techniques because they reduce the amount of damage to surrounding tissue and muscle and the amount of time required to complete the procedure. During minimally invasive surgery (MIS), percutaneous extensions are often mated to or formed on a bone anchor. The extensions can be used for guiding tools and devices to the implant site and, once the bone anchor is implanted, the extensions can be manipulated from outside of the patient's body to distract or compress vertebrae. However, these manipulation steps can be cumbersome as the working area tends to be crowded and the required instrumentation can be difficult to position, adjust, and/or maintain at a desired location. The size and configuration of the extensions can also make it difficult to achieve the desired compression or distraction, particularly when the extensions intersect and are criss-crossed.
Accordingly, there is a need for improved methods and devices for manipulating one or more bone anchors implanted in a patient's spine.